MEMBRANE PROCESSES
DAVID C. SAMMON
Chemistry Division, Atomic Energy Research Establishment,
Harwell, Didcot, Oxfordshire, UK
ABSTRACT
Membrane processes can be used to treat a variety of aqueous wastes. In some
applications the aim is to limit pollution of the environment, in others, wastes
are purified to produce water suitable for re-use and there are several examples
where valuable byproducts are extracted. Electrical, chemical potential and
pressure gradients are used in conjunction with semi-permeable membranes
to effect separations in aqueous solutions. Reverse osmosis, ultrafiltration,
electrodialysis and transport depletion are all beginning to be employed to
process wastes from industries such as pulp and paper, dairy, and metal
finishing and also to treat sewage and polluted surface waters.
iNTRODUCTION
A membranc can be defined as a barrier separating two fluids. The barriers
considered here do not prevent the passage of all species but are permeable
to some and impermeable to others. Such membranes are termed semipermeable and usually are in the form of thin sheets of polymeric material.
Since the amount of a species transported across a membrane is inversely
proportional to the thickness, it is advantageous to have the thinnest mem-
brane possible. In practice, considerations such as mechanical strength
usually determine the lower limit of membrane thickness. In many cases
synthetic polymers are used and many have been developed specially to
provide the required semi-permeable characteristics.
TYPES OF MEMBRANE PROCESS
In order to drive a species through a membrane permeable to that species
a force is required. Such forces are listed in Table 1 along with processes in
which these forces are employed. The types of separations possible are also
listed. In all these processes the membranes separate two liquid phases. In
a similar way, gas separations can be performed but these are outside the
scope of this paper. In Table 1 only the most important processes arc listed
though others are known or are being developed. Examples are pervaporation in which volatile species are transported by means of pressure on one
side and suction on the other side of the membrane, and piezodialysis
where ionic solutes are transported under the effect of pressure. The latter
is thus in some ways the opposite of reverse osmosis.
From Table I the potential of such processes is obvious in that various
423
DAVID C. SAM MON
Table I. Driving forces in membrane processes
Force
Hydrostatic pressure
Potential difference
Concentration
Species not passing*
through membrane
Species passing
Process
through membrane
ultrafiltration
solvent
solvent and small
solutes
solutcs
larger solutes
electrodialysis
ionic solutes
solvent, 11011-ionic
transport depletion
tc cations. n solvent,
n small solutes
solutes
all others, larger
solutes
osmosis
solvent
solutes
dialysis
solvent, small
solutes
larger solutes
reverse osmosis
difference
* There are two possible reasons for a sotnte not passing throngh the membrane
Ii) membrane impermeable
(ii) force does not cause species to move
'c cation permeable membrane
uncharged porous membrane
separations are possible such as removing solutes from solution and separating solutes. The fundamentals of four of these processes will be outlined
and the ways in which they are being used in waste treatment will be described.
REVERSE OSMOSIS'
In reverse osmosis or hyperfiltration as it is sometimes called the membrane
has to be permeable to the solvent, water, and impermeable to solutes. and
pressure is used to drive the water through the membrane. The name reverse
Osmosis
Equilibrium
Solvent
Flow of solvent
Reverse osmosis
Solution
No flow of solvent
Flen of solvent
Figure 1. Osmosis and reverse osmosis
osmosis arises from the fact that the direction of solvent flow is the reverse
to that occurring in osmosis, as illustrated in Figure 1. The process has been
424
MEMBRANE PROCESSES
widely studied as a means of desalting brackish or sea-water. In practice.
useful membranes are not totally impermeable to salt and some compromise
has to be reached between flux of water and rejection of salt. The most
extensively used membrane material is cellulose acetate though an aromatic
polyamide is also used. Thin films—say 10-20 ji thick--of cellulose acetate
do not have a high enough permeate flux at reasonable pressures of 30-40
bar to make the process economically attractive but asymmetric membranes
have been developed as illustrated in Figure 22. The membrane is a very
Active layer
Intermediate Layer
-
Port
of porous layer
(1OO thick)
Figure 2. Cross section of membrane
0
E
E
x
a)
0
80 90 95
98
50
Percentage salt rejection
99
Figure 3. Performance of cellulose acetate membrane at 40 bar on 5000 ppm. NaCI
425
DAVID C. SAMMON
thin semipermeable layer O.25x thick on a much thicker non-selective highly
porous support. The performance of such a membrane is illustrated in
Figure 3 and the inter-relationship between flux and rejection is clear.
The water flux is proportional to the difference between applied pressure
and osmotic pressure whereas salt transport is almost independent of pres-
sure. Thus increasing the pressure results in higher water flux and salt
rejection but unfortunately can lead to a reduction in membrane life: Cellulose acetate membranes can be used over a pH range of 3 to 8 outside this
range hydrolysis leads to loss of salt rejection. There is a clear need for more
stable membranes particularly for applications other than desalting.
Reverse osmosis membranes reject species other than salts. For cellulose
acetate the following criteria can be used
(i) the higher the ionic charge, the higher the rejection.
(ii) the lower the tendency to form hydrogen bonds. the higher the
rejection;
(iii) the larger the molecule, the higher the rejection.
Solutes such as urea are poorly rejected whereas sugars are well rejected.
In reverse osmosis plant the feed is pumped past the membrane. Water
passes through and solute is left behind at the surface. Liquid at the surface
is much less well mixed than in the bulk and the solute concentration at
the surface increases until the arrival rate is equalled by the combined effects
of diffusion and mixing. This concentration polarization results in a reduced
salt rejection and can lead to the precipitation of sparingly soluble salts. It
Porous tube
Feed flaw
Cellulose
ocetote
Perforated
PVC
baftli
membrane
Membrane
Graaved phenotic
Paper
'-Permeate
substrate
support plate
PLATE AND FRAME
LARGE TUBE
Feed
Roll to
Hallow fibres
ossembte
flow
.
e
<V*
Per meot(aut",
Permeate side—."
•
backing material
with membrane on '
each side and glued
oround edges ond to
centre tube
>
rinc
Praduct
water
HOLLOW FINE FIBRES
Permeate flow
(after passage
through membrane)
'-
SPIRAL WOUND
liqure 4. Types of reverse osmosis module'
426
MEMBRANE PROCESSES
can bc reduced by more effective mixing but at the expense of pumping
power.
Prcssures used vary from 30 bar to more than 60 bar. the latter being
needed for feed solutions with high osmotic pressure such as sea-water
(approximately 30 bar). The membranes have to be supported against this
pressure and various types of module have been developed as shown in
Figure 4.
The different versions can be summarized as follows.
(i) Flat sheets in plate and frame or spiral wrap.
(ii) Membranes inside porous tubes.
(iii) Membranes on the outside of porous supports.
(iv) Hollow fibres that need no support.
Plants with capacity in excess of 1000 m3 per day are now in use. These
are of modular construction and frequently some form of pretreatment is
needed to protect the reverse osmosis units from chemical effects or fouling
by suspended material.
ULTRAFILTRATION
Membranes are available which reject no salt but do reject larger molecules and colloidal species. These are used in ultrafiltration and the important
parameter is the molecular weight cut-off. i.e. the limiting molecular weight
beyond which molecules do not pass through the membrane. Values range
between 500 and 100000 and a wide range of membranes is available. These
are made from various polymers and some are chemically very stable. They
are, in effect, polymeric sheets with carefully controlled pore size and.
usually, some degree of asymmetry. Typical applications are the fractionation
of protein species or the separation of macromolecules from smaller solutes
such as sugars.
The membranes used offer much less resistance to the passage of water
and much lower pressures can be used (2 to 10 bars). Indeed increasing the
pressure frequently achieves nothing as a layer of the rejected species is
formed on the membrane and increases in thickness if one attempts to force
more water through the membrane. This deposited layer thus limits the flux
through the membrane.
The equipment used is similar to that for reverse osmosis though lighter
in construction because of the lower pressures. Flat sheet units and modules
with membranes on the inside of tubes are most commonly used.
ELECTRODIALYSIS
In eleetrodialysis, ions move through membranes under the influence of
a potential gradient. Two types of membrane are used; one is permeable to
cations and impermeable to anions and vice versa for the other. The membranes are made of ion-exchanging materials similar in some ways to those
used in cation and anion-exchange beads, The two types are arranged
alternately as shown in Figure 5. In commercial plant many more compartments are arranged in parallel. Ions move as shown with the net result that
427
PAC 37 3 F
DAVID C. SAMMON
the ionic concentration in one set of alternate compartments is decreased
in the other set it is increased. It is normal to supply the electrode compart-
ments from a separate solution in order to protect the electrodes from
injurious species in the feedwater,
The ions transported carry the electrical current and thus increasing the
current results in an increased removal of ions. However, concentration
polarization occurs here too. The liquid in the centre of a compartment can
be stirred effectively whereas that next to the membrane is virtually static.
Since anions are transported more readily through the anion membrane
than through the solution, the concentration of anions in the layer next to
the cathode side of the anion membrane can fall until it becomes comparable
with the concentration of hydroxyl ions. Such a low concentration zone has
high electrical resistance and leads to higher energy consumption. A'so the
transport of hydroxyl ions through the anion membrane effectively wastes
current and can cause precipitation of basic salts which may be deposited
on the membranes. Another problem occurs when the feed contains large
organic anions since these become trapped in the anion membranes and
alter the semi-permeability.
Descitted product
Anode
Cut hode
Feed wuter
iiriure 5. Desalting by electrodialysis. A is an anion-permeable memhrane ('. a cation-permeable
membrane
In addition to these problems there are two overall limitations to the
process. The ions that are transported take some water molecules with
428
MEMBRANE PROCESSES
them this is called electro-osmosis and limits the degree of concentration
possible. If the desalted stream has too low a concentration. e.g. less than
400 ppm., the electrical resistance becomes too high for the economic
transport of ions.
Electrodialysis plant is made up of a stack of membranes and spacers as
shown in Figure 6. The whole is clamped together and flow through the two
sets of compartments is through the duct and into the compartment via the
appropriate port. Each compartment is thus made up of a spacer. two
membranes and a screen to separate the membranes and provide a degree
of turbulence. Much ingenuity has gone into the design of spacer screen. the
prevention of liquid and electrical leaks, and reducing the compartment
thickness to decrease electrical resistance. Plants with capacity greater than
1000 m3 are being used for the desalination of brackish water.
Anion-
Colion-
Mony more
membrones
spocer fromes,
ond onother
end frome
Figure 6. Exploded view of electrodialysis stack4
TRANSPORT DEPLETION
Some of the problems encountered in electrodialysis are eliminated in the
process called transport depletion where the anion membrane is replaced by
a neutral porous film as shown in Figure 7. Concentration and desalting
still occur but at lower current efficiency. No concentration gradients are
established close to the neutral membranes and fouling and precipitation do
not occur. With appropriate choice of pore size of the neutral membrane
the transport of larger anions can be prevented.
The equipment used for this process is the same as that for electro-
dialysis.
Various other processes are being studied. These include electrogravitation where electrical transport is combined with gravitational transport due
to density gradients.
429
DAVID C. SAMMON
1 ij
Transport depletion process N is a neutral. porous membrane C. a cation-permeable
membrane
APPLICATION TO WASTES
The ideal treatment of an aqueous waste would result in water of quality
adequate for re-use or safe discharge. recovery of all useful byproducts and
an absolute minimum of harmless materials to be released to the environment. Membrane processes are not particularly cheap to operate. For the
large scale application to desalination of brackish water. costs are 30p
Sterling/l0(X) gallons treated (or 0.4 DM per m3). The costs are. of course.
higher for smaller scale more specialized applications and this means that.
at the moment, there is emphasis either on recovering valuable products or
on handling difficult wastes. Moreover, contaminants are not destroyed by
membrane processes, they are merely transferred. l-lowei'er. as the cost of
process water increases there will be more incentive to use these processes
to produce water suitable for recycling.
Some areas of application are listed in Table 2 along with the current
status as derived from published literature. This list is obviously out of date
now but it does give an impression of the range of applications and of whether
these are being practised on a full production scale.
In general. membrane processes offer the possibility of separating water
from vanous types of solute and of separating solutes either on the basis of
size or because some are ionized and others are not. In addition to these
cases where a high degree of separation is achieved. there are many instances
where the composition of the dissolved material is altered. One example is
430
MEMBRANE PROCESSES
Table 2. Application of membrane processes in waste treatment
Waste
Processes considered
Current status
Pulp and paper mill effluents
ED, Ta RO. UF
RO 200 m3 per day pilot5
Sewage
RO. UF
UF I 2(1 m3 per day
production5
Rinses from electrophoretic
UF
UF production8
ED. RO. UF
ED laboratory tests°
UF laboratory tests'°
RO 0 m3 per day pilot7
painting
Effluent from metal-finishing
plating
RO l000m3perday"
Acid mine drainage waters
ED. TD. RO + OF
Whey treatment
ED Eleetrodialysis
TO Transport depiction
RO 50 m3 per day pilot'2
ED. T.D 10 m3 per day
production'3
RO + UF 10 m3 per day
production'4
R() Reverse osmosis
tJ F Ultrafiltration
in reverse osmosis where the permeate has a considerably reduced salt
content and an increased ratio of uni- to multi-valent ions.
WHEY TREATMENT
Whey is the byproduct of cheese-making and, since it contains about
five per cent of lactose, it has a very high biological oxygen demand. it has
some value as a pig food but considerable quantities are discharged to waste
and the lactose content causes problems. Whey also contains protein of high
nutritive value and inorganic salts; various membrane processes are used
to recover useful products and to alleviate the pollution problem. These are
listed in Table 3. Removal of inorganic salts prior to drying yields a product
that is suitable for use in a number of foods. This demineralization is
accomplished by electrodialysis or transport depletion and both processes
are in use in the USA.
With electrodialysis the high calcium content can result in the formation
of precipitates on the cation membranes while protein fractions can be
Thhle 3 Processes used in the treatment of whey
Process used
Effect required
Partial removal of water
Almost total removal of water
Removal of salts
Removal of lactose and salts
Evaporation or RO
Above + spray drying
ED or TD
UF
431
PAC-—37--3
0
DAVID C. SAMMON
deposited on the anion membranes. Operation at low current density and
high liquid velocity is used to control precipitation of calcium salts and
polarity reversal is an effective means of limiting the deposition of proteins.
In addition, the inorganic deposits can be removed by washing with acid.
and alkaline cleaning solutions proteinaceous fouling.
These problems do not occur with transport depletion and though. under
ideal conditions, the current efficiency of this process is only about half that
in electrodialysis, the difference largely disappears in the treatment of whey.
The whey can be processed at its starting concentration of six per cent
solids or after prior concentration which results in lower energy consumption.
The compromise between amount of salt removed and processing cost
usually results in 60—70 per cent salt removal.
Protein
concentrate
Whey
Water
lactose
salts
Lact ose
con centrake
Low BOD
water
Fiqure 8. Dual process for treating whey
Simple removal of water from whey is not much used. Instead ultrafiltration alone or in conjunction with reverse osmosis is preferred. The dual
process is depicted in Figure 8 and yields concentrates of proteins and lactose
and water of low enough BOD for disposal without further treatment. In
ultrafiltration, water, lactose and salts are removed in equal percentages and
products with a range of protein to lactose ratios can be made as shown in
Table 4. The product with ratio of 0.67 has a composition close to that of
skim milk powder and can be used as a substitute for this. Products with
different ratios could be used in a number of ways but the markets for such
products are not yet developed.
The ultrafiltrate contains a large fraction of the lactose and this can be
extracted using reverse osmosis. This sugar has a number of uses but, if it
were extracted from all the whey produced, new markets would have to be
identified. Plants for the fractionation of whey are operating in Europe and
the USA. Flux can be maintained by using suitable cleaning procedures and
adequate sanitary standards can be achieved.
432
MEM8RANE PROCESSES
Table 4. Composition of whey protein concentrate
water removed
Protein to lactose
ratio in dry solids
total solids
—
0
6.2
0.14
50
75
90
95
6.9
8.2
11.7
17.3
0.28
0.56
1.40
2.80
SEWAGE TREATMENT
The treatment of whey is already performed in small plants whereas the
treatment of domestic sewage would generally require very large plants.
The steps used in conventional sewage treatment are outlined in Figure 9
and reverse osmosis has been applied at each stage in pilot-scale trials,
Considerable fouling occurs leading to a reduction of permeate flux but the
magnitude of this effect decreases as more pretreatment stages are used.
Raw
sewage
Preliminary treatment
Screening, comminution
Preliminary treated
sewage
Primary treatment
Sedimentation
Settled
sewage
Percolating fitter,
activated sludge
Secondary treatment
Secondary
effluent
Activated carbonS
Tertiary treatment
sand filtration
Tertiary
effluent
Other treatments
Fiqure 9. Steps in conventional treatment of sewage
433
DAVID C. SAM MON
Various trials on plants of capacity up to 70 m3 per day have been reported
and considerable progress has been made in devising suitable cleaning
methods. Periodic flushing with enzyme detergent is particularly successful
and typical results are shown in Figure JO.
The verflcat Hnes indicate when the
equipment was cleaned with BIZ'
8C
0
I__
100
Fiqurr' Jo
200
300
1.00
500
600
Duration of test h
Treatment of secondary sewage cifluent by R()
This application is at too early a stage in development for reliable cost
estimates to be available. Indeed there is considerable discussion as to the
most useful role reverse osmosis can play in sewage treatment. Possibilities
vary from the extraction of water for purposes such as horticulture to
reduction in salt content of the effluent. The latter role is one particularly
suited to reverse osmosis and will be required as the salinity of rivers
increases. In addition to salt removal, high removals of BOD and COD arc
obtained by reverse osmosis. Microbiological species such as bacteria and
viruses do not pass through the membranes.
WASTES FROM METAL FINISHING
Wastes from electroplating, etching and pickling can be. in principle.
processed by electrodiafysis, reverse osmosis and ultrafiltration. Various
laboratory scale studies have been reported and recently the use of reverse
osmosis in a zero discharge system has been described. The process is used
in two ways; the first is to extract chromium from rinse baths and return it
to an electroplating solution 30 m3 per day in the second the effluent
from a flocculation process is treated to produce water suitable for recycle
434
MEMBRANE PROCESSES
4000 m3 per day-—and a concentrate that can be used, after drying. to
de-ice roads. The value of the recovered chromium counterbalances the
costs of the recovery process.
In a similar way 99 per cent of the nickel can be recovered from rinses
subsequent to plating in Watts or sulphamate baths. The limited pH stability
of cellulose acetate membranes is a disadvantage in many plating applications.
Electrodialysis has also been investigated as a means of recovering nickel
in electroplating. Amongst other possibilities is the maintenance of a constant
zinc concentration in an etch bath by removing excess by electrodialysis.
Membrane processes are more likely to be used where the recovered
species can be re-used easily. This does not apply for pickle effluents and
precipitation is preferred unless there is a high quality effluent required for
recycling as in the example quoted earlier.
EFFLUENT FROM ELECTROPHORETIC PAINTING
Electrophoretic painting is used extensively in the automotive industry
as well as for domestic appliances and office furniture. The paint is basically
an aqueous solution of a resin solubilized by alkali and to which pigments
and organic solvents are added. After electrodeposition, undeposited paint
is washed off before the deposited paint is cured. The undeposited material
can represent a drag-out loss as high as 50 per cent which is unacceptable in
terms of both loss of paint and production of high BOD effluent. Ultrafiltration has been used extensively to solve these problems. It would be
possible simply to concentrate paint from the water rinse and return it to
the dip tank but this is not the most useful procedure. Impurities gradually
build up in the paint dip tank and these, particularly those that are ionic in
character, adversely affect electrodeposition. In addition. the use of water
alone can cause instability and precipitation in the material that is washed
off, Thus it is preferable to use an integrated system as shown in Figure 11.
In this the contents of the dip tank are treated by ultrafiltration when the
higher molecular weight species such as paint and pigments are separated
from water, ionic impurities and organic solvents. The paint and pigments
are returned to the dip tank, some of the ultrafiltrate is bled off to control
the level of impurities and the rest is used for washing off excess paint. The
solvents exercise a stabilizing influence on the paint and are more effective
than water alone for washing.
The throwing power of the paint is improved at high paint concentrations
and the use of ultrafiltration encourages this. This application imposes
fairly severe conditions on the membrane namely pH up to 9.5, high content
of solids, some of them abrasive, and the presence of organic solvents. The
first plants in use on the production line were marketed by Dorr Oliver but
now equipment from various manufacturers is employed.
CONCLUSION
Various other applications in waste treatment have been studied but these
few will suffice to indicate the usefulness of the membrane processes described.
435
DAVID C. SAMMON
At the moment, membrane processes are beginning to play an increasing
role in waste treatment and it is clear that this increase will continue as industry becomes more familiar with relatively new processes such as reverse
osmosis. Tighter control on pollution and demand for water exceeding
available supply will further accelerate the use of membrane processes.
Waste
Fiqure 11. Usc of UF in electrophoretic painting
REFERENCES
MIT Press . Cambridge. Mass
U Merten. Dea1ipiation by R''er.w'
2 G. J Gittens. P. A Hitchcock. D. C. Sammon and G. E. Wakely. De.aIiuuiim. 8. 369 (1970)
J. E. Cruver. \iarine !echnoloqv. 9. 216 (1972)
R. E. Lacey and S. Loeb. Indu,tria/ Proce.inq wit/i \Ien,hrune.s. p 46 Wiley Interscience
New York (1972).
A. J. Wiley. K. Scharpf. 1. Bansal and D. Arps. Thppi.55, 1671 (1972).
P. L. Stavenger. ('hem. Lnqnq Proqr. 67, 30(1971)
I. Nushaum. J. H. Sleigh and S. S. Kremen. !"WQA Report. WPCRS 1704() 05/70 (1970)
F. Forbes. (/iem. Process Enqnq.. p 49 (December 1970).
D P. McDonald. Enqng Mat De.. 17. 23 (1973).
if) R
L. Goldsmith. R. P. de Filippi and S. Hossain. Paper presented at 70th National Meeting
of A.I.Chem.E. Atlantic City. New Jersey (1971).
J. Schrantz. lndu.st. I'inisliinq. 49 3( (1973),
12 S. S. Kremer, A. B. Riedinger et al. US Office of Saline Water Progr. Rep. No. 586 (1970).
Reference 4. page 57.
' I. K Nielsen. A. G. Bundgaard. 0. J. Olsen and R. F. Madsen. Prm'ess !3,ocliem.. p 7 (September 1972).
436